The Isles of Scilly Biodiversity Audit 2008 - Cornwall Wildlife Trust

2.2.1.33 Marine mammals – whales,dolphins, porpoises and sealsOverviewThe marine mammals of the Isles of Scilly includelarge and small cetaceans (whales, dolphins andporpoises) and also seals. These charismatic andpopular species are important to the economy of theislands for tourism and play important roles in marineecosystems.Cetaceans are divided into baleen whales (Mysticeti),which use plates of baleen to filter out food from thewater column and toothed whales (Odontoceti) whichhave teeth. Dolphins and porpoises are included in theOdontoceti.As an oceanic archipelago, Scilly is visited by oceanicand coastal marine mammals. Some cetaceans areknown here only from records of a single stranding,while for others the waters around the islandsrepresent an important habitat. All cetaceans usingthese waters are so mobile that the waters aroundScilly only represent part of their home ranges (N.Tregenza, pers. comm.).Grey seals Halichoerus grypus are one of the bestknown species that wildlife lovers associate with theIsles of Scilly. They congregate in one area to mate anduse known breeding areas where they are able to ‘haulout’ on the rocks. Scilly is therefore an important sitefor breeding seals. Satellite tracking of grey seals hasshown that they forage over very large distances andtagging shows that seals from Brittany and Wales visitthe Isles of Scilly at times (Westcott, 1993).The grey seal population in Scilly is about 350 strongand is resident there throughout the year althoughsome individuals may travel between Scilly and themainland (Lambert, 2000).Cetaceans are often seen by passengers on the RMVScillonian passenger ferry as they travel between StMary’s and Penzance. Harbour porpoises Phocoenaphocoena are frequently sighted when the water is calmenough for their small dorsal fins to be visible as theybreak the surface, and may live there throughout theyear. Several different species of dolphin includingbottlenose dolphins Tursiops truncatus and commondolphins Delphinus delphis are seen regularly,particularly in the summer months when many boatusers are out on the water.Important AreasMost cetacean sightings records for the Isles of Scillyare to the south of the Islands around Bishop’s RockLighthouse, Poll Bank and Giants Castle but this maysimply reflect the fact that there are more birding andfishing boat trips in this area.The population of grey seals is distributed betweenthe Eastern Isles, the Norrad Rocks and the WesternRocks.Conservation ImportanceAll cetaceans are BAP species, managed by a groupedspecies plan. Cetaceans are protected by the BernConvention (1979) which is implemented in the UKthrough the Wildlife and Countryside Act (1981). Theyare also covered by the Bonn Convention (1979) whichprotects migratory species.The harbour porpoise used to be seen daily inScillonian waters until the 1950s. They are still seen inScilly in the calmer summer months and but they arefar less common than they were in the past.Common dolphins prefer deep water and areoccasionally sighted off the Isles of Scilly. They are theBottlenose dolphins Tursiops truncatus are occasionally observed in the waters around Scilly. Photo: Frank GloysteinIsles of Scilly biodiversity audit Page 50